1. Field of the Invention
The present invention relates to an image transmission control method, and more particularly, to an image transmission control method for a facsimile apparatus having a means for executing transmission procedures in a shorter time in order to reduce a time required for communication.
2. Description of the Related Art
FIG. 1 shows a diagram explaining a basic communication protocol applied to communications between transmitting and receiving facsimile apparatus in accordance with ITU-T Recommendation. Referring specifically to FIG. 1, signals sent between the transmitting side and the receiving side during a facsimile transmission include a CED signal 1 serving as a called station identification signal sent from the receiving side over a network; signals NSF, CSI, DIS generally designated by 203 for respectively declaring a non-standard function, a receiving capability and the telephone number of a called station, i.e., the receiving side; a TSI signal for transmitting the telephone number of the transmitting side and a DCS signal for setting a transmission function, together designated by 204; a TCF signal 208 for continuously sending "0" for 1.4 seconds at an image signal transmission rate specified by a calling terminal, i.e., the transmitting side to determine whether or not the specified image signal transmission rate is appropriate; a CFR signal 209 indicating that the TCF signal 208 has been correctly received; a pix signal 5 for delivering image signal data; an EOP signal 6 indicating that the last page has been sent; an MCF signal 7 indicating that the immediately previous page has been received without errors; and a DCN signal 201 for declaring that the communication has been completed and that the network connection is disconnected.
If the TCF signal 208 cannot be correctly received due to a bad network condition, the receiving side sends an FTT signal instead of the CFR signal 209 which would otherwise be transmitted to the transmitting side. In response, the transmitting side utilizes the TSI/DCS signal 204 to inform the receiving side of a new transmission rate and then sends the TCF signal 208. This procedure is repeated until the CFR signal 209 is returned from the receiving side.
FIG. 2 is a diagram explaining a high level data link control (HDLC) frame constituting the above-mentioned respective signals. In FIG. 2, the HDLC frame includes a preamble 211 consisting of sequential one-second flag codes; an address code 212; a control code 213; a facsimile control field 214; a facsimile information field 215; a frame check sequence 216; and one or more flags 217. The HDLC frame, composed of these elements, is generally sent at a transmission rate of 300 bps.
Next, the operation of the facsimile apparatus will be described with reference again to FIG. 1. In FIG. 1, the receiving side, upon receiving a call, sends a CED signal indicating that this is a non-audio terminal. The CED signal is a single tone signal at 2100 Hz which is generated for more than 2.6 seconds and less than 4.0 seconds.
In continuation, the receiving side sends the NSF signal used for identifying an option which may be different depending on respective manufacturers, a CSI signal including a country code, a telephone number, and so on, and a DIS signal used for identifying a standard function in accordance with ITU-T Recommendation. These signals are each formatted in accordance with the HDLC frame shown in FIG. 2. Also, signals TSI, DCS, CFR, EOP, MCF, DCN are likewise formatted in accordance with the HDLC frame. The HDLC frame sends binary code information after approximately one second of preamble 211 for establishing synchronization between modems (modulation-demodulation units) of the transmitting side and the receiving side. The binary code information includes substantially fixed information such as the address code 212, control code 213, and flag 217 as well as rather variable information such as the facsimile control field 214 indicating the kind of control signal, the facsimile information field 215 indicating detailed contents thereof, and the frame check sequence 216 for checking the frame. Also, when a plurality of HDLC frames are sequentially sent such as a sequence of NSF/CSI/DIS signals, these HDLC frames share the preamble 211.
In response to the NSF/CSI/DIS signals 203 sent from the receiving side, the transmitting side determines the capabilities of the respective functions of the receiving side, and informs the receiving side of the determined capabilities through the TSI/DCS signal 204.
Subsequently, the TCF signal 208 is sent for about 1.5 seconds to prepare the receiving side to receive a high rate signal. When conditions are appropriate the receiving side returns the CFR signal 209 to enable transmission of image data (pix signal 5) at a high rate to be proceeded with. When conditions are not appropriate, the reception side returns the FTT signal to step down the signal rate of the image data signal (for example, from 14.4 KBPS to 12 KBPS), and the above-mentioned procedure is repeated by sending a further TCF signal 208.
After the pix signal 5 has been transmitted from the transmitting side and received by the receiving side, a post-procedure protocol is entered. First, the transmitting side sends the EOP signal 6 indicative of the last page. The receiving side, in response to the EOP signal 6, returns the MCF signal 7 to the transmitting side which next sends the DCN signal 210 informing the receiving side of a disconnected network, thus completing the communication.
Since a conventional facsimile apparatus is constituted as described above, it takes approximately 15 to 16 seconds to complete the procedure required to send an image signal (pix signal 5) after a call has been answered. Assuming that an image signal is sent at a transmission rate of 14.4 KBPS, the procedure requires a time period of two to three times that of transmission of a document (pix signal 5), i.e., the original purpose. In other words, network utilization efficiency is extremely low, resulting in increased communication costs.
Also, when a network condition is not appropriate for transmission, the transmission rate is changed until the CFR signal 209 is returned from the receiving side in order to determine an appropriate transmission rate. However, approximately five seconds are required for the procedure each time the transmission rate is changed.
Japanese Patent Public Disclosure No. 5-219334 (1993) has proposed a solution to the above-mentioned problems. Specifically, a transmitting side sends a tone signal to a receiving side to reduce the number of procedures required to transmit an image to the receiving side. However, the disclosed solution does not address the reduction of procedures required after the transmission of an image signal (pix signal 5) nor to the provision of a shorter preamble in the HDLC frame. Since this solution doe not reduce the time required to execute the procedures before and after the transmission of the image signal (pix signal 5), the network utilization efficiency cannot be improved sufficiently.